<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>CSS Expressive Generalizations and Gadgetry Level 1</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
  <meta content="exploring" name="csswg-work-status">
  <meta content="ED" name="w3c-status">
  <meta content="This module extends the vocabulary of CSS with terms frequently used in certain domains, making it easier for authors to write understandable and maintainable style sheets." name="abstract">
  <link href="../default.css" rel="stylesheet" type="text/css">
  <link href="../csslogo.ico" rel="shortcut icon" type="image/x-icon">
<style>
  body {
    background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat;
  }
  </style>
  <link href="https://www.w3.org/TR/css-egg/" rel="canonical">
<style type="text/css">
body {
	background: -o-double-rainbow(center), url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat;
	background: double-rainbow(center), url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat;
}
:any-link {
	cursor: -o-double-rainbow(center), pointer;
	cursor: double-rainbow(center), pointer;
}
</style>
<style>
/**
This would be nicer using attr() or custom properties,
but there’s not enough support in browsers to make a good demo with them.

The proportions to make this fit in the spec are as follow:
1px = 1ls
1s = 10 days
sun radius x 2
planets & moon radius x 100
planetary orbits x 0.25
moon orbit x 5

All are rounded up to the closest integer pixel,
because otherwise it doesn’t work right in some browsers.
 */
#sun {
	border-width: 5px;
	margin: -5px;
	border-color: yellow;
	top: 50%;
	box-shadow: yellow 0 0 10px, yellow 0 0 20px, yellow 0 0 30px;
	/* The box shadow size has no other justification than making the sun look good. */
}
#mercury {
	border-width: 1px;
	margin: -1px;
	top: -49px;
	transform-origin: 1px 50px;
	-webkit-transform-origin: 1px 50px;
	animation-duration: 8.797s;
	-webkit-animation-duration: 8.797s;
	border-color: lightgray;
}
#venus {
	border-width: 2px;
	margin: -2px;
	top: -90px;
	transform-origin: 2px 92px;
	-webkit-transform-origin: 2px 92px;
	animation-duration: 22.47s;
	-webkit-animation-duration: 22.47s;
	border-color: #FFD881;
}
#earth {
	border-width: 2px;
	margin: -2px;
	top: -125px;
	transform-origin: 2px 127px;
	-webkit-transform-origin: 2px 127px;
	animation-duration: 36.53s;
	-webkit-animation-duration: 36.53s;
	border-color: blue;
}
#moon {
	border-width: 1px;
	margin: -1px;
	top: -6px;
	transform-origin: 1px 7px;
	-webkit-transform-origin: 1px 7px;
	animation-duration: 2.953s;
	-webkit-animation-duration: 2.953s;
	border-color: #E0E0E0;
}
#mars {
	border-width: 1px;
	margin: -1px;
	top: -190px;
	transform-origin: 1px 191px;
	-webkit-transform-origin: 1px 191px;
	animation-duration: 68.7s;
	-webkit-animation-duration: 68.7s;
	border-color:#AD674D;
}
#space {
	height:400px;
	background:black;
	overflow:hidden;
	position: relative;
}
.celestial-body {
	left: 50%;
	position:absolute;
	border-style:solid;
	border-radius: 50%;
	animation: orbit linear infinite;
	-webkit-animation: orbit linear infinite;
}
@keyframes orbit {
	from { transform: rotate(0); }
	to { transform: rotate(-360deg); }
}
@-webkit-keyframes orbit {
	from { -webkit-transform: rotate(0); }
	to { -webkit-transform: rotate(-360deg); }
}
	</style>
<style>
.fabulous {
background: radial-gradient(closest-corner circle at bottom, transparent, rgba(250, 250, 210, 0.251) 63%, rgba(148, 0, 211, 0.251), rgba(0, 0, 128, 0.251), rgba(0, 0, 255, 0.251), rgba(0, 128, 0, 0.251), rgba(255, 255, 0, 0.251), rgba(255, 165, 0, 0.251), rgba(255, 0, 0, 0.251), transparent 67%, transparent 90%, rgba(255, 0, 0, 0.126), rgba(255, 165, 0, 0.126), rgba(255, 255, 0, 0.126), rgba(0, 128, 0, 0.126), rgba(0, 0, 255, 0.126), rgba(0, 0, 128, 0.126), rgba(148, 0, 211, 0.126), transparent 100%);
max-width: 500px;
display:block;
margin-left: auto;
margin-right: auto;
}
</style>
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
<style>/* style-dfn-panel */

.dfn-panel {
    position: absolute;
    z-index: 35;
    height: auto;
    width: -webkit-fit-content;
    width: fit-content;
    max-width: 300px;
    max-height: 500px;
    overflow: auto;
    padding: 0.5em 0.75em;
    font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
    background: #DDDDDD;
    color: black;
    border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0; }
.dfn-panel li { list-style: inside; }
.dfn-panel.activated {
    display: inline-block;
    position: fixed;
    left: .5em;
    bottom: 2em;
    margin: 0 auto;
    max-width: calc(100vw - 1.5em - .4em - .5em);
    max-height: 30vh;
}

.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-syntax-highlighting */

.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"><a class="logo" href="https://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
   <h1 class="p-name no-ref" id="title">CSS Expressive Generalizations and Gadgetry Level 1</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <details>
    <summary>Specification Metadata</summary>
    <div data-fill-with="spec-metadata">
     <dl>
      <dt>This version:
      <dd><a class="u-url" href="https://drafts.csswg.org/css-egg/">https://drafts.csswg.org/css-egg/</a>
      <dt>Latest published version:
      <dd><a href="https://www.w3.org/TR/css-egg/">https://www.w3.org/TR/css-egg/</a>
      <dt class="editor">Editor:
      <dd class="editor p-author h-card vcard" data-editor-id="43241"><a class="p-name fn u-url url" href="https://florian.rivoal.net">Florian Rivoal</a> (<span class="p-org org">Invited Expert</span>)
      <dt>Suggest an Edit for this Spec:
      <dd><a href="https://github.com/w3c/csswg-drafts/blob/master/css-egg-1/Overview.bs">GitHub Editor</a>
      <dt>Issue Tracking:
      <dd><a href="https://github.com/w3c/csswg-drafts/labels/css-egg-1">GitHub Issues</a>
     </dl>
    </div>
   </details>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="https://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 <a href="https://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="https://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="https://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="https://www.keio.ac.jp/">Keio</a>, <a href="https://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="https://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document">permissive document license</a> rules apply. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This module extends the vocabulary of CSS with terms frequently used in certain domains, making it easier for authors to write understandable and maintainable style sheets.</p>
    <a href="https://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc. 
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This is a public copy of the editors’ draft.
	It is provided for discussion only and may change at any moment.
	Its publication here does not imply endorsement of its contents by W3C.
	Don’t cite this document other than as work in progress. </p>
   <p> <a href="https://github.com/w3c/csswg-drafts/issues">GitHub Issues</a> are preferred for discussion of this specification.
	When filing an issue, please put the text “css-egg” in the title,
	preferably like this:
	“[css-egg] <i data-lt>…summary of comment…</i>”.
	All issues and comments are <a href="https://lists.w3.org/Archives/Public/public-css-archive/">archived</a>,
	and there is also a <a href="https://lists.w3.org/Archives/Public/www-style/">historical archive</a>. </p>
   <p> This document was produced by the <a href="https://www.w3.org/Style/CSS/">CSS Working Group</a>. </p>
   <p> This document was produced by a group operating under
	the <a href="https://www.w3.org/Consortium/Patent-Policy/">W3C Patent Policy</a>.
	W3C maintains a <a href="https://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group;
	that page also includes instructions for disclosing a patent.
	An individual who has actual knowledge of a patent which the individual believes contains <a href="https://www.w3.org/Consortium/Patent-Policy/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/Consortium/Patent-Policy/#sec-Disclosure">section 6 of the W3C Patent Policy</a>. </p>
   <p> This document is governed by the <a href="https://www.w3.org/2019/Process-20190301/" id="w3c_process_revision">1 March 2019 W3C Process Document</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#intro"><span class="secno">1</span> <span class="content"> Introduction</span></a>
     <ol class="toc">
      <li><a href="#interact"><span class="secno">1.1</span> <span class="content"> Module Interactions</span></a>
     </ol>
    <li>
     <a href="#units"><span class="secno">2</span> <span class="content"> Extended Units</span></a>
     <ol class="toc">
      <li><a href="#astro-units"><span class="secno">2.1</span> <span class="content"> Astronomical units</span></a>
      <li><a href="#traditional-time"><span class="secno">2.2</span> <span class="content"> Traditional time units </span></a>
      <li>
       <a href="#speech"><span class="secno">2.3</span> <span class="content"> Speech rate</span></a>
       <ol class="toc">
        <li><a href="#voice-rate-ext"><span class="secno">2.3.1</span> <span class="content"> Extension to the <span class="property">voice-rate</span> property</span></a>
       </ol>
     </ol>
    <li><a href="#rainbow"><span class="secno">3</span> <span class="content"> Double Rainbow</span></a>
    <li><a href="#acknowledgments"><span class="secno"></span> <span class="content"> Acknowledgments</span></a>
    <li>
     <a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
     <ol class="toc">
      <li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
      <li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
      <li>
       <a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
       <ol class="toc">
        <li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
        <li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
        <li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
       </ol>
     </ol>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a>
    <li><a href="#issues-index"><span class="secno"></span> <span class="content">Issues Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content"> Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p><em>This section is not normative.</em></p>
   <p>As evidenced by the extraordinary variety of designs found on the web,
	CSS is a very expressive language,
	capable of describing unlimited styles and layouts.
	However, one of the design goal of CSS was also to make it
	easy to learn and to read by people.
	Making it simple for untrained authors to read and guess
	the meaning of a style sheet after only a basic introduction to CSS
	has contributed greatly to the success of the language.</p>
   <p>In addition, easy-to-read style sheets are also easier to maintain.</p>
   <p>In this spirit, this specification introduces a few new values and units,
	which, while not bringing significant new capabilities to the web platform,
	makes it possible to write style sheets that are easier to read than ever before,
	by enabling authors to use values and units that are familiar
	and appropriate for the domain they are working on.</p>
   <h3 class="heading settled" data-level="1.1" id="interact"><span class="secno">1.1. </span><span class="content"> Module Interactions</span><a class="self-link" href="#interact"></a></h3>
   <p>This module extends:</p>
   <ul>
    <li>the data type definitions in <a data-link-type="biblio" href="#biblio-css3-values">[CSS3-VALUES]</a> 
    <li>the <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-images-4/#typedef-gradient" id="ref-for-typedef-gradient">&lt;gradient></a> definition in <a data-link-type="biblio" href="#biblio-css4-images">[CSS4-IMAGES]</a> 
    <li>the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-speech-1/#voice-rate" id="ref-for-voice-rate">voice-rate</a> property in <a data-link-type="biblio" href="#biblio-css3speech">[CSS3SPEECH]</a> 
   </ul>
   <h2 class="heading settled" data-level="2" id="units"><span class="secno">2. </span><span class="content"> Extended Units</span><a class="self-link" href="#units"></a></h2>
    CSS provides several <a data-link-type="dfn" href="https://drafts.csswg.org/css-values-3/#absolute-length" id="ref-for-absolute-length">absolute length</a> units such as <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#cm" id="ref-for-cm">cm</a> or <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#in" id="ref-for-in">in</a>,
as well as several <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#time-value" id="ref-for-time-value" title="Expands to: s | ms">&lt;time></a> units such as <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#s" id="ref-for-s">s</a> or <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#ms" id="ref-for-ms">ms</a> They can all be converted between each other at fixed ratios, but are
nonetheless provided in recognition that it is most convenient for authors
to use units that they are most familiar with, or that some designs are
more naturally expressed in some units than others. 
   <h3 class="heading settled" data-level="2.1" id="astro-units"><span class="secno">2.1. </span><span class="content"> Astronomical units</span><a class="self-link" href="#astro-units"></a></h3>
    This specification adds four <span id="ref-for-absolute-length①">absolute length</span> units
which should be appreciated by authors
in fields such as astronomy and fundamental physics.
The new units are <span class="css">ls</span>, <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#pc" id="ref-for-pc">pc</a> and their sub multiples <span class="css">pls</span> and <span class="css">apc</span>.
They are defined as <a data-link-type="dfn" href="https://drafts.csswg.org/css-values-3/#physical-units" id="ref-for-physical-units">physical units</a>. 
   <table class="data">
    <thead>
     <tr>
      <th>unit 
      <th>name 
      <th>equivalence 
    <tbody>
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="apc">apc<a class="self-link" href="#apc"></a></dfn> 
      <td>attoparsec 
      <td>1 <span class="css">apc</span> = 3.086 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#cm" id="ref-for-cm①">cm</a> 
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="pls">pls<a class="self-link" href="#pls"></a></dfn> 
      <td>picolightsecond 
      <td>1 <span class="css">pls</span> = 1.133 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#px" id="ref-for-px">px</a> 
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="ls">ls<a class="self-link" href="#ls"></a></dfn> 
      <td>lightsecond 
      <td>1 <span class="css">ls</span> = 10<sup>12</sup> <span class="css">pls</span> 
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="pc">pc<a class="self-link" href="#pc"></a></dfn> 
      <td>parsec 
      <td>1 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#pc" id="ref-for-pc①">pc</a> = 10<sup>18</sup> <span class="css">apc</span> 
   </table>
   <p class="note" role="note"><span>Note:</span> Lightsecond was chosen instead of the more familiar lightyear
to make sure that layouts computed during leap years
would be unambiguously identical
to those computed during non-leap years.</p>
   <p class="note" role="note"><span>Note:</span> This specification does not address relativistic effects
of scrolling or animations and transitions
at (or beyond) the speed of light.</p>
   <div class="example" id="celestial-css">
    <a class="self-link" href="#celestial-css"></a> When writing a page representing the inner solar system at scale, the following markup and style could be used. 
<pre><code class="lang-markup highlight"><c- p>&lt;</c-><c- f>div</c-> <c- e>id</c-><c- o>=</c-><c- s>space</c-><c- p>></c->
  <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>sun</c-> <c- e>data-color</c-><c- o>=</c-><c- s>yellow</c->
    <c- e>data-radius</c-><c- o>=</c-><c- s>2.3ls</c-><c- p>></c->
    <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>mercury</c-> <c- e>data-color</c-><c- o>=</c-><c- s>lightgray</c->
      <c- e>data-radius</c-><c- o>=</c-><c- s>0.0081ls</c-> <c- e>data-semimajor-axis</c-><c- o>=</c-><c- s>193ls</c-> <c- e>data-orbital-period</c-><c- o>=</c-><c- s>6.278ftn</c-><c- p>>&lt;/</c-><c- f>div</c-><c- p>></c->
    <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>venus</c-> <c- e>data-color</c-><c- o>=</c-><c- s>#FFD881</c->
      <c- e>data-radius</c-><c- o>=</c-><c- s>0.020ls</c-> <c- e>data-semimajor-axis</c-><c- o>=</c-><c- s>361ls</c-> <c- e>data-orbital-period</c-><c- o>=</c-><c- s>16.05ftn</c-><c- p>>&lt;/</c-><c- f>div</c-><c- p>></c->
    <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>earth</c-> <c- e>data-color</c-><c- o>=</c-><c- s>blue</c->
      <c- e>data-radius</c-><c- o>=</c-><c- s>0.021ls</c-> <c- e>data-semimajor-axis</c-><c- o>=</c-><c- s>499ls</c-> <c- e>data-orbital-period</c-><c- o>=</c-><c- s>26.09ftn</c-><c- p>></c->
      <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>moon</c-> <c- e>data-color</c-><c- o>=</c-><c- s>#E0E0E0</c->
        <c- e>data-radius</c-><c- o>=</c-><c- s>0.0057ls</c-> <c- e>data-semimajor-axis</c-><c- o>=</c-><c- s>1.28ls</c-> <c- e>data-orbital-period</c-><c- o>=</c-><c- s>2.109ftn</c-><c- p>>&lt;/</c-><c- f>div</c-><c- p>></c->
    <c- p>&lt;/</c-><c- f>div</c-><c- p>></c->
    <c- p>&lt;</c-><c- f>div</c-> <c- e>class</c-><c- o>=</c-><c- s>celestial-body</c-> <c- e>id</c-><c- o>=</c-><c- s>mars</c-> <c- e>data-color</c-><c- o>=</c-><c- s>#AD674D</c->
      <c- e>data-radius</c-><c- o>=</c-><c- s>0.011ls</c-> <c- e>data-semimajor-axis</c-><c- o>=</c-><c- s>760ls</c-> <c- e>data-orbital-period</c-><c- o>=</c-><c- s>49.07ftn</c-><c- p>>&lt;/</c-><c- f>div</c-><c- p>></c->
<c- p>&lt;/</c-><c- f>div</c-><c- p>>&lt;/</c-><c- f>div</c-><c- p>></c->
<c- p>&lt;</c-><c- f>style</c-><c- p>></c-></code><code class="lang-css highlight"><c- f>#space </c-><c- p>{</c->
  <c- k>background</c-><c- p>:</c->black<c- p>;</c->
  <c- k>overflow</c-><c- p>:</c->hidden<c- p>;</c->
  <c- k>position</c-><c- p>:</c-> relative<c- p>;</c->
<c- p>}</c->
<c- f>.celestial-body </c-><c- p>{</c->
  <c- k>position</c-><c- p>:</c->absolute<c- p>;</c->
  <c- k>left</c-><c- p>:</c-> <c- m>50</c-><c- l>%</c-><c- p>;</c-> <c- k>top</c-><c- p>:</c-> <c- m>50</c-><c- l>%</c-><c- p>;</c->
  <c- k>border-radius</c-><c- p>:</c-> <c- m>50</c-><c- l>%</c-><c- p>;</c->
  <c- k>border-style</c-><c- p>:</c->solid <c- nf>attr</c-><c- p>(</c->data-radius length<c- p>,</c-> <c- m>0</c-><c- p>)</c-> <c- nf>attr</c-><c- p>(</c->data-color color<c- p>,</c-> white<c- p>);</c->
  <c- k>margin</c-><c- p>:</c-> <c- nf>calc</c-><c- p>(</c-><c- m>-1</c-> * attr(data-radius length, 0));
  transform-origin: attr(data-radius length, 0)
                    calc(attr(data-radius length, 0) + attr(data-semimajor-axis length, 0));
  animation: orbit linear infinite attr(data-orbital-period time, 0);
}
@keyframes orbit {
  from { transform: translateY(calc(attr(data-semimajor-axis length, 0) * -1)) rotate(0); }
  to { transform: translateY(calc(attr(data-semimajor-axis length, 0) * -1)) rotate(-360deg); }
}
</code><code class="lang-markup highlight"><c- p>&lt;/</c-><c- f>style</c-><c- p>></c->
</code>
</pre>
    <p>The following is a disappointingly small sample rendering (not at scale)
	of what this would look like.
	With the new units and more space than this specification can offer,
	this would instead be overwhelmingly awesome.</p>
    <div id="space">
     <div class="celestial-body" id="sun">
      <div class="celestial-body" id="mercury"></div>
      <div class="celestial-body" id="venus"></div>
      <div class="celestial-body" id="earth">
       <div class="celestial-body" id="moon"></div>
      </div>
      <div class="celestial-body" id="mars"></div>
     </div>
    </div>
   </div>
   <h3 class="heading settled" data-level="2.2" id="traditional-time"><span class="secno">2.2. </span><span class="content"> Traditional time units </span><a class="self-link" href="#traditional-time"></a></h3>
   <p>While the <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#s" id="ref-for-s①">s</a> and <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#ms" id="ref-for-ms①">ms</a> units defined in <a data-link-type="biblio" href="#biblio-css3-values">[CSS3-VALUES]</a> are appropriate for very short durations,
their use to express longer time spans is inadequate.
Moreover, authors may sometimes wish to use less dry terminology
and prefer more expressive and vivid language. This specification
therefore introduces two <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#time-value" id="ref-for-time-value①" title="Expands to: s | ms">&lt;time></a> units: <span class="css">ftn</span> and its subdivision <span class="css">mftn</span>.</p>
   <table class="data">
    <thead>
     <tr>
      <th>unit 
      <th>name 
      <th>equivalence 
    <tbody>
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="ftn">ftn<a class="self-link" href="#ftn"></a></dfn> 
      <td>Fortnight 
      <td>1 <span class="css">ftn</span> = 1209600 <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-values-4/#s" id="ref-for-s②">s</a> 
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="mftn">mftn<a class="self-link" href="#mftn"></a></dfn> 
      <td>Millifortnight 
      <td>1 <span class="css">ftn</span> = 1000 <span class="css">mftn</span> 
   </table>
   <h3 class="heading settled" data-level="2.3" id="speech"><span class="secno">2.3. </span><span class="content"> Speech rate</span><a class="self-link" href="#speech"></a></h3>
   <p>This specification introduces an new <a data-link-type="dfn" href="https://drafts.csswg.org/css-values-4/#dimension" id="ref-for-dimension">dimension</a>,
together with a new unit to be used with this dimension.
The <dfn class="dfn-paneled css" data-dfn-type="type" data-export id="speech-rate-value"><a class="production css" data-link-type="type" href="#speech-rate-value" id="ref-for-speech-rate-value">&lt;speech-rate></a></dfn> described the speed at which a text is read aloud.
It can be expressed using the <span class="css">tmbl</span> unit, as defined below.</p>
   <table class="data">
    <thead>
     <tr>
      <th>unit 
      <th>name 
      <th>definition 
    <tbody>
     <tr>
      <th><dfn data-dfn-type="dfn" data-export id="tmbl">tmbl<a class="self-link" href="#tmbl"></a></dfn> 
      <td>Timble 
      <td>This is the rate of speech of Sir Tim Berners-Lee
		        when delivering a speech. 
   </table>
   <p class="note" role="note"><span>Note:</span> Authors are advised to be cautious when using this unit,
as it can easily lead to rates of speech that are too high
for the listener to follow. <span class="css">1tmbl</span> is generally considered to be about
the maximum rate a listener without special training can sustainably understand
while paying close attention.
Consider reducing the speed when the audience is distracted, tired,
or comprises many non-native speakers.
These factors are cumulative.
For dictation or during meetings for which live minutes are taken,
a rate of 0.3tmbl is more appropriate.</p>
   <p class="issue" id="issue-6104d49e"><a class="self-link" href="#issue-6104d49e"></a> For languages other than English, should <span class="css">1tmbl</span> be defined as
the same number of words per minute as <span class="css">1tmbl</span> in English?
This seems problematic as we do not have a precise definition for <em>word</em>,
and because average word length can vary significantly per language.
Alternatively, defining <span class="css">1tmbl</span> as
the speed at which Tim Berners-Lee speaks in that language
is also problematic,
given that there are languages that He does not speak at all.
Tim Berners-Lee has provided an alternative definition of the timble, <a href="http://schepers.cc/timble">as documented by Doug Schepers</a>: <q>speech at more than 1 timble is difficult to understand;
speech below 1 timble is simply boring</q>.
Doug himself defines it as <q>the uppermost rate of speech
at which a normal person
can understand what’s being said in their native language</q>.</p>
   <h4 class="heading settled" data-level="2.3.1" id="voice-rate-ext"><span class="secno">2.3.1. </span><span class="content"> Extension to the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-speech-1/#voice-rate" id="ref-for-voice-rate①">voice-rate</a> property</span><a class="self-link" href="#voice-rate-ext"></a></h4>
   <p>This specification extends the <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-speech-1/#voice-rate" id="ref-for-voice-rate②">voice-rate</a> property, so that the <span class="css">tmbl</span> unit can be used.</p>
   <table class="def propdef partial" data-link-for-hint="voice-rate">
    <tbody>
     <tr>
      <th>Name:
      <td><a class="css" data-link-type="property" href="https://drafts.csswg.org/css-speech-1/#voice-rate" id="ref-for-voice-rate③">voice-rate</a>
     <tr class="value">
      <th><a href="https://drafts.csswg.org/css-values/#value-defs">New values:</a>
      <td class="prod">[normal <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-one" id="ref-for-comb-one">|</a> x-slow <span id="ref-for-comb-one①">|</span> slow <span id="ref-for-comb-one②">|</span> medium <span id="ref-for-comb-one③">|</span> fast <span id="ref-for-comb-one④">|</span> x-fast <span id="ref-for-comb-one⑤">|</span> <a class="production css" data-link-type="type" href="#speech-rate-value" id="ref-for-speech-rate-value①">&lt;speech-rate></a>] <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-any" id="ref-for-comb-any">||</a> &lt;percentage>
   </table>
   <p class="issue" id="issue-670f6c64"><a class="self-link" href="#issue-670f6c64"></a> Should negative values be allowed for <a class="production css" data-link-type="type" href="#speech-rate-value" id="ref-for-speech-rate-value②">&lt;speech-rate></a> for reversed speech?</p>
   <h2 class="heading settled" data-level="3" id="rainbow"><span class="secno">3. </span><span class="content"> Double Rainbow</span><a class="self-link" href="#rainbow"></a></h2>
   <p><a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-images-4/#typedef-gradient" id="ref-for-typedef-gradient①">&lt;gradient></a> allows sophisticated visual effects,
but they are tedious to write, review and maintain
as non trivial gradients cannot easily be visualised by merely reading the source.
This specification introduces a new syntax to describe a particular kind of <a class="production css" data-link-type="function" href="https://drafts.csswg.org/css-images-3/#funcdef-radial-gradient" id="ref-for-funcdef-radial-gradient">&lt;radial-gradient()></a>: double rainbows.
This spectacular visual effect which would improve most web pages
is currently underused due to the difficulty of specifying it correctly.</p>
   <p>The <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-images-4/#typedef-gradient" id="ref-for-typedef-gradient②">&lt;gradient></a> syntax is extended to accept <a class="production css" data-link-type="function" href="#funcdef-double-rainbow" id="ref-for-funcdef-double-rainbow">&lt;double-rainbow()></a> in addition to the other values defined in <a data-link-type="biblio" href="#biblio-css4-images">[CSS4-IMAGES]</a></p>
<pre><dfn class="dfn-paneled" data-dfn-type="function" data-export id="funcdef-double-rainbow">double-rainbow()</dfn> = double-rainbow(
   <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#typedef-position" id="ref-for-typedef-position">&lt;position></a> [, [ <a class="production" data-link-type="type" href="#typedef-extent" id="ref-for-typedef-extent">&lt;extent></a> | <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#length-value" id="ref-for-length-value" title="Expands to: em | vb | ch | cm | vh | vi | in | ex | vw | ic | pt | px | lh | pc | rem | rlh | vmax | advance measure | vmin | mm | cap | q">&lt;length></a> | <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#percentage-value" id="ref-for-percentage-value">&lt;percentage></a>]]?
)
<dfn class="dfn-paneled" data-dfn-type="type" data-noexport id="typedef-extent"><a class="production" data-link-type="type" href="#typedef-extent" id="ref-for-typedef-extent①">&lt;extent></a></dfn> = <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-images-3/#valdef-size-closest-corner" id="ref-for-valdef-size-closest-corner">closest-corner</a> | <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-images-3/#valdef-size-closest-side" id="ref-for-valdef-size-closest-side">closest-side</a> | <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-images-3/#valdef-size-farthest-corner" id="ref-for-valdef-size-farthest-corner">farthest-corner</a> | <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-images-3/#valdef-size-farthest-side" id="ref-for-valdef-size-farthest-side">farthest-side</a>
</pre>
   <p><a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#length-value" id="ref-for-length-value①" title="Expands to: em | vb | ch | cm | vh | vi | in | ex | vw | ic | pt | px | lh | pc | rem | rlh | vmax | advance measure | vmin | mm | cap | q">&lt;length></a> or <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-3/#percentage-value" id="ref-for-percentage-value①">&lt;percentage></a> gives the radius of the outermost circle of the double rainbow explicitly.
Percentages values are relative to the corresponding dimension of the gradient box.
Negative values are invalid.</p>
   <p>If the second argument is omitted, the default value is <a class="css" data-link-type="maybe" href="https://drafts.csswg.org/css-images-3/#valdef-size-farthest-side" id="ref-for-valdef-size-farthest-side①">farthest-side</a>.</p>
   <div class="example" id="rainbow-unicorn">
    <a class="self-link" href="#rainbow-unicorn"></a> This feature was initially introduced by Opera Software.
The following page, when viewed in Opera (between version 11.60 and 12.16),
demonstrates tasteful use of double rainbows: <a href="http://media.opera.com/media/press/2011/unicorn/">http://media.opera.com/media/press/2011/unicorn/</a> 
    <p>By combining <a class="css" data-link-type="maybe" href="#funcdef-double-rainbow" id="ref-for-funcdef-double-rainbow①">double-rainbow()</a> with <a href="https://fetch.spec.whatwg.org/#unicorn">about:unicorn</a> <a data-link-type="biblio" href="#biblio-fetch">[FETCH]</a>,
modern web standards can be used to achieve a similar effect.</p>
<pre><code class="lang-markup highlight"><c- p>&lt;</c-><c- f>img</c-> <c- e>src</c-><c- o>=</c-><c- s>"about:unicorn"</c->
<c- e>style</c-><c- o>=</c-><c- s>"background: double-rainbow(bottom, closest-corner)"</c-><c- p>></c->
</code></pre>
    <p><img class="fabulous" src="./unicorn.svg"></p>
   </div>
   <h2 class="no-num heading settled" id="acknowledgments"><span class="content"> Acknowledgments</span><a class="self-link" href="#acknowledgments"></a></h2>
   <p>Thanks especially to the following people for their ideas, feedback, and experimental implementations (in alphabetical order by first name):
Anne van Kesteren,
Bruce Lawson,
Chris Mills,
Doug Schepers,
Daniel Glazman,
Leif Arne Storset,
Paul Verbeek,
Peter Linss,
Tim Berners-Lee (assuming the quotation by Doug correctly records what Tim said).</p>
  </main>
  <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
  <h3 class="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-conventions"></a></h3>
  <p>Conformance requirements are expressed with a combination of
	descriptive assertions and RFC 2119 terminology. The key words “MUST”,
	“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
	“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
	document are to be interpreted as described in RFC 2119.
	However, for readability, these words do not appear in all uppercase
	letters in this specification. </p>
  <p>All of the text of this specification is normative except sections
	explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
  <p>Examples in this specification are introduced with the words “for example”
	or are set apart from the normative text with <code>class="example"</code>,
	like this: </p>
  <div class="example" id="example-ae2b6bc0">
   <a class="self-link" href="#example-ae2b6bc0"></a> 
   <p>This is an example of an informative example. </p>
  </div>
  <p>Informative notes begin with the word “Note” and are set apart from the
	normative text with <code>class="note"</code>, like this: </p>
  <p class="note" role="note">Note, this is an informative note. </p>
  <p>Advisements are normative sections styled to evoke special attention and are
	set apart from other normative text with <code>&lt;strong class="advisement"></code>, like
	this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
  <h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
  <p>Conformance to this specification
	is defined for three conformance classes: </p>
  <dl>
   <dt>style sheet 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
			style sheet</a>. 
   <dt>renderer 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that interprets the semantics of a style sheet and renders
			documents that use them. 
   <dt>authoring tool 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that writes a style sheet. 
  </dl>
  <p>A style sheet is conformant to this specification
	if all of its statements that use syntax defined in this module are valid
	according to the generic CSS grammar and the individual grammars of each
	feature defined in this module. </p>
  <p>A renderer is conformant to this specification
	if, in addition to interpreting the style sheet as defined by the
	appropriate specifications, it supports all the features defined
	by this specification by parsing them correctly
	and rendering the document accordingly. However, the inability of a
	UA to correctly render a document due to limitations of the device
	does not make the UA non-conformant. (For example, a UA is not
	required to render color on a monochrome monitor.) </p>
  <p>An authoring tool is conformant to this specification
	if it writes style sheets that are syntactically correct according to the
	generic CSS grammar and the individual grammars of each feature in
	this module, and meet all other conformance requirements of style sheets
	as described in this module. </p>
  <h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
  <p>The following sections define several conformance requirements
		for implementing CSS responsibly,
		in a way that promotes interoperability in the present and future. </p>
  <h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
  <p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
		(and <a href="http://www.w3.org/TR/CSS2/conform.html#ignore">ignore as appropriate</a>)
		any at-rules, properties, property values, keywords, and other syntactic constructs
		for which they have no usable level of support</strong>.
		In particular, user agents <em>must not</em> selectively ignore
		unsupported property values and honor supported values in a single multi-value property declaration:
		if any value is considered invalid (as unsupported values must be),
		CSS requires that the entire declaration be ignored. </p>
  <h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
  <p>To avoid clashes with future stable CSS features,
		the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
  <h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>
  <p>Once a specification reaches the Candidate Recommendation stage,
		implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> implementation
		of any CR-level feature they can demonstrate
		to be correctly implemented according to spec,
		and should avoid exposing a prefixed variant of that feature. </p>
  <p>To establish and maintain the interoperability of CSS across
		implementations, the CSS Working Group requests that non-experimental
		CSS renderers submit an implementation report (and, if necessary, the
		testcases used for that implementation report) to the W3C before
		releasing an unprefixed implementation of any CSS features. Testcases
		submitted to W3C are subject to review and correction by the CSS
		Working Group. </p>
  <p>
   Further information on submitting testcases and implementation reports
		can be found from on the CSS Working Group’s website at <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
		Questions should be directed to the <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a> mailing list. 
<script src="https://www.w3.org/scripts/TR/2016/fixup.js"></script>
  </p>
  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#apc">apc</a><span>, in §2.1</span>
   <li><a href="#funcdef-double-rainbow">double-rainbow()</a><span>, in §3</span>
   <li><a href="#typedef-extent">&lt;extent></a><span>, in §3</span>
   <li><a href="#ftn">ftn</a><span>, in §2.2</span>
   <li><a href="#ls">ls</a><span>, in §2.1</span>
   <li><a href="#mftn">mftn</a><span>, in §2.2</span>
   <li><a href="#pc">pc</a><span>, in §2.1</span>
   <li><a href="#pls">pls</a><span>, in §2.1</span>
   <li><a href="#speech-rate-value">&lt;speech-rate></a><span>, in §2.3</span>
   <li><a href="#tmbl">tmbl</a><span>, in §2.3</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-typedef-position">
   <a href="https://drafts.csswg.org/css-values-4/#typedef-position">https://drafts.csswg.org/css-values-4/#typedef-position</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-typedef-position">3. 
Double Rainbow</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-cm">
   <a href="https://drafts.csswg.org/css-values-4/#cm">https://drafts.csswg.org/css-values-4/#cm</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-cm">2. 
Extended Units</a>
    <li><a href="#ref-for-cm①">2.1. 
Astronomical units</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dimension">
   <a href="https://drafts.csswg.org/css-values-4/#dimension">https://drafts.csswg.org/css-values-4/#dimension</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dimension">2.3. 
Speech rate</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-in">
   <a href="https://drafts.csswg.org/css-values-4/#in">https://drafts.csswg.org/css-values-4/#in</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-in">2. 
Extended Units</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-ms">
   <a href="https://drafts.csswg.org/css-values-4/#ms">https://drafts.csswg.org/css-values-4/#ms</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-ms">2. 
Extended Units</a>
    <li><a href="#ref-for-ms①">2.2. 
Traditional time units </a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-pc">
   <a href="https://drafts.csswg.org/css-values-4/#pc">https://drafts.csswg.org/css-values-4/#pc</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-pc">2.1. 
Astronomical units</a> <a href="#ref-for-pc①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-px">
   <a href="https://drafts.csswg.org/css-values-4/#px">https://drafts.csswg.org/css-values-4/#px</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-px">2.1. 
Astronomical units</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-s">
   <a href="https://drafts.csswg.org/css-values-4/#s">https://drafts.csswg.org/css-values-4/#s</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-s">2. 
Extended Units</a>
    <li><a href="#ref-for-s①">2.2. 
Traditional time units </a> <a href="#ref-for-s②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-comb-one">
   <a href="https://drafts.csswg.org/css-values-4/#comb-one">https://drafts.csswg.org/css-values-4/#comb-one</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-comb-one">2.3.1. 
Extension to the voice-rate property</a> <a href="#ref-for-comb-one①">(2)</a> <a href="#ref-for-comb-one②">(3)</a> <a href="#ref-for-comb-one③">(4)</a> <a href="#ref-for-comb-one④">(5)</a> <a href="#ref-for-comb-one⑤">(6)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-comb-any">
   <a href="https://drafts.csswg.org/css-values-4/#comb-any">https://drafts.csswg.org/css-values-4/#comb-any</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-comb-any">2.3.1. 
Extension to the voice-rate property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-valdef-size-closest-corner">
   <a href="https://drafts.csswg.org/css-images-3/#valdef-size-closest-corner">https://drafts.csswg.org/css-images-3/#valdef-size-closest-corner</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-size-closest-corner">3. 
Double Rainbow</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-valdef-size-closest-side">
   <a href="https://drafts.csswg.org/css-images-3/#valdef-size-closest-side">https://drafts.csswg.org/css-images-3/#valdef-size-closest-side</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-size-closest-side">3. 
Double Rainbow</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-valdef-size-farthest-corner">
   <a href="https://drafts.csswg.org/css-images-3/#valdef-size-farthest-corner">https://drafts.csswg.org/css-images-3/#valdef-size-farthest-corner</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-size-farthest-corner">3. 
Double Rainbow</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-valdef-size-farthest-side">
   <a href="https://drafts.csswg.org/css-images-3/#valdef-size-farthest-side">https://drafts.csswg.org/css-images-3/#valdef-size-farthest-side</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-size-farthest-side">3. 
Double Rainbow</a> <a href="#ref-for-valdef-size-farthest-side①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-funcdef-radial-gradient">
   <a href="https://drafts.csswg.org/css-images-3/#funcdef-radial-gradient">https://drafts.csswg.org/css-images-3/#funcdef-radial-gradient</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-funcdef-radial-gradient">3. 
Double Rainbow</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-length-value">
   <a href="https://drafts.csswg.org/css-values-3/#length-value">https://drafts.csswg.org/css-values-3/#length-value</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-length-value">3. 
Double Rainbow</a> <a href="#ref-for-length-value①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-percentage-value">
   <a href="https://drafts.csswg.org/css-values-3/#percentage-value">https://drafts.csswg.org/css-values-3/#percentage-value</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-percentage-value">3. 
Double Rainbow</a> <a href="#ref-for-percentage-value①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-time-value">
   <a href="https://drafts.csswg.org/css-values-3/#time-value">https://drafts.csswg.org/css-values-3/#time-value</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-time-value">2. 
Extended Units</a>
    <li><a href="#ref-for-time-value①">2.2. 
Traditional time units </a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-absolute-length">
   <a href="https://drafts.csswg.org/css-values-3/#absolute-length">https://drafts.csswg.org/css-values-3/#absolute-length</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-absolute-length">2. 
Extended Units</a>
    <li><a href="#ref-for-absolute-length①">2.1. 
Astronomical units</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-physical-units">
   <a href="https://drafts.csswg.org/css-values-3/#physical-units">https://drafts.csswg.org/css-values-3/#physical-units</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-physical-units">2.1. 
Astronomical units</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-voice-rate">
   <a href="https://drafts.csswg.org/css-speech-1/#voice-rate">https://drafts.csswg.org/css-speech-1/#voice-rate</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-voice-rate">1.1. 
Module Interactions</a>
    <li><a href="#ref-for-voice-rate①">2.3.1. 
Extension to the voice-rate property</a> <a href="#ref-for-voice-rate②">(2)</a> <a href="#ref-for-voice-rate③">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-typedef-gradient">
   <a href="https://drafts.csswg.org/css-images-4/#typedef-gradient">https://drafts.csswg.org/css-images-4/#typedef-gradient</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-typedef-gradient">1.1. 
Module Interactions</a>
    <li><a href="#ref-for-typedef-gradient①">3. 
Double Rainbow</a> <a href="#ref-for-typedef-gradient②">(2)</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[css-values-4]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-typedef-position" style="color:initial">&lt;position></span>
     <li><span class="dfn-paneled" id="term-for-cm" style="color:initial">cm</span>
     <li><span class="dfn-paneled" id="term-for-dimension" style="color:initial">dimension</span>
     <li><span class="dfn-paneled" id="term-for-in" style="color:initial">in</span>
     <li><span class="dfn-paneled" id="term-for-ms" style="color:initial">ms</span>
     <li><span class="dfn-paneled" id="term-for-pc" style="color:initial">pc</span>
     <li><span class="dfn-paneled" id="term-for-px" style="color:initial">px</span>
     <li><span class="dfn-paneled" id="term-for-s" style="color:initial">s</span>
     <li><span class="dfn-paneled" id="term-for-comb-one" style="color:initial">|</span>
     <li><span class="dfn-paneled" id="term-for-comb-any" style="color:initial">||</span>
    </ul>
   <li>
    <a data-link-type="biblio">[css3-images]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-valdef-size-closest-corner" style="color:initial">closest-corner</span>
     <li><span class="dfn-paneled" id="term-for-valdef-size-closest-side" style="color:initial">closest-side</span>
     <li><span class="dfn-paneled" id="term-for-valdef-size-farthest-corner" style="color:initial">farthest-corner</span>
     <li><span class="dfn-paneled" id="term-for-valdef-size-farthest-side" style="color:initial">farthest-side</span>
     <li><span class="dfn-paneled" id="term-for-funcdef-radial-gradient" style="color:initial">radial-gradient()</span>
    </ul>
   <li>
    <a data-link-type="biblio">[CSS3-VALUES]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-length-value" style="color:initial">&lt;length></span>
     <li><span class="dfn-paneled" id="term-for-percentage-value" style="color:initial">&lt;percentage></span>
     <li><span class="dfn-paneled" id="term-for-time-value" style="color:initial">&lt;time></span>
     <li><span class="dfn-paneled" id="term-for-absolute-length" style="color:initial">absolute length</span>
     <li><span class="dfn-paneled" id="term-for-physical-units" style="color:initial">physical units</span>
    </ul>
   <li>
    <a data-link-type="biblio">[CSS3SPEECH]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-voice-rate" style="color:initial">voice-rate</span>
    </ul>
   <li>
    <a data-link-type="biblio">[CSS4-IMAGES]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-typedef-gradient" style="color:initial">&lt;gradient></span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-css-values-4">[CSS-VALUES-4]
   <dd>Tab Atkins Jr.; Elika Etemad. <a href="https://www.w3.org/TR/css-values-4/">CSS Values and Units Module Level 4</a>. 31 January 2019. WD. URL: <a href="https://www.w3.org/TR/css-values-4/">https://www.w3.org/TR/css-values-4/</a>
   <dt id="biblio-css3-images">[CSS3-IMAGES]
   <dd>Elika Etemad; Tab Atkins Jr.. <a href="https://www.w3.org/TR/css3-images/">CSS Image Values and Replaced Content Module Level 3</a>. 17 April 2012. CR. URL: <a href="https://www.w3.org/TR/css3-images/">https://www.w3.org/TR/css3-images/</a>
   <dt id="biblio-css3-values">[CSS3-VALUES]
   <dd>Tab Atkins Jr.; Elika Etemad. <a href="https://www.w3.org/TR/css-values-3/">CSS Values and Units Module Level 3</a>. 31 January 2019. CR. URL: <a href="https://www.w3.org/TR/css-values-3/">https://www.w3.org/TR/css-values-3/</a>
   <dt id="biblio-css3speech">[CSS3SPEECH]
   <dd>Daniel Weck. <a href="https://www.w3.org/TR/css3-speech/">CSS Speech Module</a>. 5 June 2018. NOTE. URL: <a href="https://www.w3.org/TR/css3-speech/">https://www.w3.org/TR/css3-speech/</a>
   <dt id="biblio-css4-images">[CSS4-IMAGES]
   <dd>Tab Atkins Jr.; Elika Etemad; Lea Verou. <a href="https://www.w3.org/TR/css-images-4/">CSS Image Values and Replaced Content Module Level 4</a>. 13 April 2017. WD. URL: <a href="https://www.w3.org/TR/css-images-4/">https://www.w3.org/TR/css-images-4/</a>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-fetch">[FETCH]
   <dd>Anne van Kesteren. <a href="https://fetch.spec.whatwg.org/">Fetch Standard</a>. Living Standard. URL: <a href="https://fetch.spec.whatwg.org/">https://fetch.spec.whatwg.org/</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="property-index"><span class="content">Property Index</span><a class="self-link" href="#property-index"></a></h2>
  <p>No properties defined.</p>
  <h2 class="no-num no-ref heading settled" id="issues-index"><span class="content">Issues Index</span><a class="self-link" href="#issues-index"></a></h2>
  <div style="counter-reset:issue">
   <div class="issue"> For languages other than English, should <span class="css">1tmbl</span> be defined as
the same number of words per minute as <span class="css">1tmbl</span> in English?
This seems problematic as we do not have a precise definition for <em>word</em>,
and because average word length can vary significantly per language.
Alternatively, defining <span class="css">1tmbl</span> as
the speed at which Tim Berners-Lee speaks in that language
is also problematic,
given that there are languages that He does not speak at all.
Tim Berners-Lee has provided an alternative definition of the timble, <a href="http://schepers.cc/timble">as documented by Doug Schepers</a>: <q>speech at more than 1 timble is difficult to understand;
speech below 1 timble is simply boring</q>.
Doug himself defines it as <q>the uppermost rate of speech
at which a normal person
can understand what’s being said in their native language</q>.<a href="#issue-6104d49e"> ↵ </a></div>
   <div class="issue"> Should negative values be allowed for <a class="production css" data-link-type="type" href="#speech-rate-value">&lt;speech-rate></a> for reversed speech?<a href="#issue-670f6c64"> ↵ </a></div>
  </div>
  <aside class="dfn-panel" data-for="speech-rate-value">
   <b><a href="#speech-rate-value">#speech-rate-value</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-speech-rate-value">2.3. 
Speech rate</a>
    <li><a href="#ref-for-speech-rate-value①">2.3.1. 
Extension to the voice-rate property</a> <a href="#ref-for-speech-rate-value②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="funcdef-double-rainbow">
   <b><a href="#funcdef-double-rainbow">#funcdef-double-rainbow</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-funcdef-double-rainbow">3. 
Double Rainbow</a> <a href="#ref-for-funcdef-double-rainbow①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="typedef-extent">
   <b><a href="#typedef-extent">#typedef-extent</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-typedef-extent">3. 
Double Rainbow</a> <a href="#ref-for-typedef-extent①">(2)</a>
   </ul>
  </aside>
<script>/* script-dfn-panel */

document.body.addEventListener("click", function(e) {
    var queryAll = function(sel) { return [].slice.call(document.querySelectorAll(sel)); }
    // Find the dfn element or panel, if any, that was clicked on.
    var el = e.target;
    var target;
    var hitALink = false;
    while(el.parentElement) {
        if(el.tagName == "A") {
            // Clicking on a link in a <dfn> shouldn't summon the panel
            hitALink = true;
        }
        if(el.classList.contains("dfn-paneled")) {
            target = "dfn";
            break;
        }
        if(el.classList.contains("dfn-panel")) {
            target = "dfn-panel";
            break;
        }
        el = el.parentElement;
    }
    if(target != "dfn-panel") {
        // Turn off any currently "on" or "activated" panels.
        queryAll(".dfn-panel.on, .dfn-panel.activated").forEach(function(el){
            el.classList.remove("on");
            el.classList.remove("activated");
        });
    }
    if(target == "dfn" && !hitALink) {
        // open the panel
        var dfnPanel = document.querySelector(".dfn-panel[data-for='" + el.id + "']");
        if(dfnPanel) {
            dfnPanel.classList.add("on");
            var rect = el.getBoundingClientRect();
            dfnPanel.style.left = window.scrollX + rect.right + 5 + "px";
            dfnPanel.style.top = window.scrollY + rect.top + "px";
            var panelRect = dfnPanel.getBoundingClientRect();
            var panelWidth = panelRect.right - panelRect.left;
            if(panelRect.right > document.body.scrollWidth && (rect.left - (panelWidth + 5)) > 0) {
                // Reposition, because the panel is overflowing
                dfnPanel.style.left = window.scrollX + rect.left - (panelWidth + 5) + "px";
            }
        } else {
            console.log("Couldn't find .dfn-panel[data-for='" + el.id + "']");
        }
    } else if(target == "dfn-panel") {
        // Switch it to "activated" state, which pins it.
        el.classList.add("activated");
        el.style.left = null;
        el.style.top = null;
    }

});
</script>